This invention generally relates to an improvement to a printing apparatus that forms an image using a plurality of exposure elements and more particularly relates to a method for improving uniformity of output prints from such a printing apparatus.
The difficulty of achieving uniform density output from a printer is a well-known problem in the printer art. Non-uniformity is particularly noticeable with high-quality color printers, where it is important to be able to faithfully reproduce subtle changes in shading and gradation or flat fields having the same density. Non-uniform response of a printhead causes unacceptable anomalies such as streaking and banding, which can easily render a print useless, or at least disappointing, for its intended audience.
Factors that contribute to printer non-uniformity vary, depending on the specific printing technology. With a thermal printhead, for example, where resistive print elements are linearly aligned along a writing surface, slight mechanical irregularities or additive mechanical tolerance variability can cause some elements to be more effective in transferring heat than others. With a printhead that scans optically, such as a CRT printhead, optical aberrations or fringe effects can mean that light power is less effectively distributed at the extreme edges of the scan pattern than it is in the center of a scan line. In a photofinishing system that uses an array of light-emitting elements, such as a Micro Light Valve Array (MLVA) in the Noritsu model QSS-2711 Digital Lab System, manufactured by Noritsu Koki Co., located in Wakayama, Japan, individual elements in the array may vary in the intensity of light emitted.
Achieving printer uniformity for high-performance printers used, for example, as photofinishing systems, graphic arts image-setters, and color proofing systems, can be particularly complex. Due to customer expectations for quality, the problem of printing apparatus non-uniformity is especially acute in the photofinishing arts. In photofinishing, the continued development of digital solutions for image scanning and printing of photographic-quality images make the problem of achieving print uniformity particularly important. To complicate the task of achieving uniformity among printers used in photofinishing, these printing apparatus may include components provided by more than one manufacturer. Companies specializing in different aspects of the photofinishing process provide exposure apparatus, development apparatus, scanning devices, film and paper, and consumable development chemicals needed in the process. In order to design a complete photofinishing printing apparatus, a systems integrator may create a system by combining preferred components and consumables from a number of vendors. In many cases, vendor companies providing the various components and consumables may even be, at least in part, competing against one another. From the perspective of a supplier of one or more components, it is advantageous to be able to provide a printing apparatus subsystem that can maintain or improve image quality with minimal dependency on other subsystems. From the alternate perspective of an integrator of components, it is advantageous to be able to purchase a necessary component or consumable from a photofinishing manufacturer as a xe2x80x9cblack boxxe2x80x9d, where no proprietary information on internal components or operation is needed or provided. Instead, in order to integrate a component or consumable into a photofinishing printing apparatus, a systems integrator only needs access to information on performance and external operation for those components.
As one relatively complex type of printer, a conventional printing apparatus used for digital photofinishing typically comprises the key subsystems shown in FIG. 1. Here, a printing apparatus is generally numbered 10. The data path for printing apparatus 10 is represented by solid arrowed line B. A digital image source 12 provides input image data. Digital image source 12 could be, for example, a color scanner. An image data manager 14 performs digital manipulation and processing of the input image data from digital image source 12. Image data manager 14 is a computer, which may be a Windows or UNIX platform, for example, specially configured for its imaging function. Image data manager 14 comprises the necessary CPU, disk storage, and memory components for processing an image and providing the image data at its output.
As the printing engine of printing apparatus 10, an image forming assembly 22 comprises a printhead 16 and support circuitry, including a transfer element 36, an optional transport mechanism 28 (where printhead 16 includes moving parts or scanning components), and a drive electronics assembly 26 that controls the amount of energy applied to transfer element 36. A system controller 30 provides control logic and processing functions for image forming assembly 22 components. Printhead 16 creates an image by applying energy from transfer element 36 onto a receiver substrate 18. For typical apparatus of this type, receiver substrate 18 is photosensitive print paper. For such a typical system, transfer element 36 applies light energy to expose the paper. Alternate combinations of receiver substrate 18 and transfer element 36 are possible, however, such as using a colorant that is applied directly to receiver substrate 18 (for example, ink) or a colorant donor material. For inkjet printing, transfer element 36 provides colorant directly, where the amount of colorant transfer is modulated by varying the amount of heat exposure energy applied to inkjet nozzles. For printing apparatus 10 using colorant donor imaging technology, transfer element 36 can apply light or heat exposure energy to a donor material (not shown) to transfer colorant to receiver substrate 18. For any type of printing apparatus 10, dashed line A represents the travel path of receiver substrate 18 from a receiver supply 24 through image forming assembly 22. A processor 20 provides any necessary processing of receiver substrate 18 in order to provide a completed output print 38. For photofinishing printing apparatus 10 that uses photosensitive silver-halide chemistry, processor 20 uses a series of chemicals (for example, bleach, fixer, and developer) that develop the latent image exposed by printhead 16 onto receiver substrate 18. For printing apparatus 10 using a donor colorant, processor 16 may transfer colorant from a receiver substrate 18 onto paper stock, with optional addition of a lamination layer.
Referring again to FIG. 1, it is instructive to note that conventional approaches for non-uniformity correction are directed to internal adjustments that are made to components within image forming assembly 22. For some types of printing apparatus 10, a sensor 58 is provided in order to measure a characteristic of transfer element 36. Sensor 58 feedback then goes to image forming assembly 22 to adjust the behavior of drive electronics assembly 26. Dotted line C represents this feedback path using sensor 58. For other types of printing apparatus 10, a scanning device 60, such as a scanner or densitometer, is employed to obtain measurements from output print 38. Data from scanning device 60 is then directed to image forming assembly 22 to adjust the behavior of drive electronics assembly 26. Dotted line D represents this alternate feedback path using scanning device 60 measurements. It is instructive to note that, when using the feedback path indicated by dotted line D, density data is obtained from output print 38. Image forming assembly 22 must perform some further conversion of this feedback density data to data values actually used by printhead 16 to control exposure.
The disclosures of the following patents illustrate conventional approaches for non-uniformity correction as applied for various types of printheads 16:
U.S. Pat. No. 5,546,165 (Rushing et al.) discloses non-uniformity correction applied in an electrostatic copier, using LED technology in transfer element 36. Feedback measurements from a scanned, flat field contone test print are obtained in order to calculate adjustments to individual LED drive currents or on-times. Referring to FIG. 1, the approach disclosed in the Rushing et al. patent modifies the behavior of drive electronics assembly 26. To obtain and adjust non-uniformity data, this approach uses the basic scanning device 60-based feedback path denoted D in FIG. 1.
U.S. Pat. No. 5,684,568 (Ishikawa et al.) discloses non-uniformity correction applied in a printer used for developing photosensitive media. Light intensity from an exposure source employing an array of lead lanthanum zirconate titanate (PLZT) light valves serves as transfer element 36. This output light is measured to identify individual light valve elements that require adjustment for non-uniformity. Referring to FIG. 1, the approach disclosed in the Ishikawa et al. patent modifies the behavior of drive electronics assembly 26 for individual light valve elements, either controlling exposure time or light power level. To obtain and adjust non-uniformity data, this approach uses the basic sensor 58-based feedback path denoted C in FIG. 1.
U.S. Pat. No. 5,997,123 (Takekoshi et al.) discloses non-uniformity correction applied in an inkjet printer, where transfer element 36 comprises an array of nozzles. Control electronics are adjusted to modify dot diameter by controlling the applied nozzle energy or by modulating the number of dots produced. Again referring to FIG. 1, the approach disclosed in the Takekoshi et al. patent modifies the behavior of drive electronics assembly 26 for individual inkjet nozzles in the printhead 16 array. To obtain and adjust non-uniformity data, this approach uses the basic scanning device 60-based feedback path denoted D in FIG. 1.
U.S. Pat. No. 6,034,710 (Kawabe et al.) discloses non-uniformity correction applied in a photofinishing printing apparatus that employs Vacuum Fluorescent Print Head (VFPH) technology for printheads 16. Again referring to FIG. 1, the approach disclosed in the Kawabe et al. patent modifies the behavior of drive electronics assembly 26 by adjusting the exposure time of individual elements in the VFPH array. To obtain and adjust non-uniformity data, this approach uses the basic sensor 58-based feedback path denoted C in FIG. 1.
U.S. Pat. No. 5,946,006 (Tajika et al.) discloses non-uniformity correction applied in an inkjet printer, where transfer element 36 comprises an array of nozzles. Referring to FIG. 1, correction data goes directly to printhead 16. To obtain and adjust non-uniformity data, this approach uses the basic scanning device 60-based feedback path denoted D in FIG. 1.
U.S. Pat. No. 5,790,240 (Ishikawa et al.) discloses non-uniformity correction applied in a printer using PLZT (or LED or LCD) printing elements as transfer element 36. Referring to FIG. 1, a correction voltage is applied directly to drive electronics assembly 26 in order to adjust the output amplitude of an individual PLZT array element. Alternately, duration of the drive signal to an individual PLZT array element is adjusted at drive electronics assembly 26. To obtain and adjust non-uniformity data, this approach uses the basic scanning device 60-based feedback path denoted D in FIG. 1.
U.S. Pat. No. 4,827,279 (Lubinsky et al.) discloses non-uniformity correction applied in a printer where printhead 16 uses an array of resistive thermal elements. Density measurements are obtained for each individual thermal element and are used to determine correction factors. Referring to FIG. 1, number of applied pulses or pulse duration at drive electronics assembly 26 are used in order to achieve uniformity. To obtain and adjust non-uniformity data, this approach uses the basic scanning device 60-based feedback path denoted D in FIG. 1.
With each of the conventional solutions noted above, non-uniformity correction is applied by making adjustments to drive electronics 26 in image forming assembly 22. This method for non-uniformity correction, however, has disadvantages, making drive electronics design more complex, requiring correction table data to be accessible at an external interface, or requiring the printhead 16 design to accommodate additional control signals. Because correction data must be fed back to image forming assembly 22, implementation of these conventional solutions requires that an integrator have detailed knowledge of the internal workings of image forming assembly 22. As noted above, this can complicate and delay commercialization of a printing apparatus, since different manufacturers may be involved. Moreover, image forming assembly 22 may not have a non-uniformity correction scheme, or may have a scheme that must be modified and improved for a specific implementation. Or, a manufacturer of image forming assembly 22 may discontinue production of a specific model, or change the design of printhead 16 components. For these reasons, it can be difficult or impossible to obtain a desired printing apparatus uniformity improvement when using the conventional methods, as illustrated in the prior art patents cited above. Thus it can be seen that conventional approaches, as outlined and illustrated by examples above, present problems that can make it difficult or impossible to obtain uniformity on an output print from a printing apparatus. While there have been methods for compensating for non-uniformity from image forming assembly 22, there is a long-felt need for a printer improvement and method for achieving uniformity on an output print.
It is an object of the present invention to provide an improvement to a printing apparatus for non-uniformity correction and a method for non-uniformity correction.
According to an aspect of the present invention, the improvement resides in a printing apparatus that uses an image forming assembly comprising a plurality of exposure elements, where the amount of exposure energy is capable of being varied at each exposure element to provide a corresponding output colorant density, the improvement comprising:
(a) an output test print formed by the image forming assembly, the test print having a predetermined set of output colorant density levels;
(b) a scanner capable of scanning the output test print and providing, to correspond to each one of the plurality of exposure elements, a plurality of scanned density data values;
(c) an image data manager that accepts a plurality of image data values from an image data source and, within each one of the predetermined set of output colorant density levels, is capable of:
(1) computing, from the plurality of scanned density data values, an exposure element average density value corresponding to each one of the plurality of exposure elements, to generate a set of exposure element average density values for the plurality of exposure elements;
(2) computing, from the set of exposure element average density values for the plurality of exposure elements, a density level average density value corresponding to the predetermined set of output colorant density levels;
(3) computing, for each one of the plurality of exposure elements, a non-uniformity correction value based on the difference between the exposure element average density value and the density level average density value;
the image data manager further capable of conditioning, corresponding to each one of the plurality of exposure elements, each of the plurality of image data values from the image data source using the non-uniformity correction value to generate a conditioned image density data value and capable of providing the conditioned image density data value to the image forming assembly.
A feature of the present invention is the adaptation of the image data manager for non-uniformity compensation.
An advantage of the present invention is that it allows an image forming assembly in the printing apparatus to be considered as a modular unit, or xe2x80x9cblack boxxe2x80x9d, so that detailed information about internal operation of the image forming assembly is not required for performing non-uniformity correction. No adjustments are made to internal components of the image forming assembly itself. Instead, the image data manager, based on scanner measurements from an output test print, directly modifies input image data provided to the image forming assembly. The present invention measures output density values to obtain a profile of printhead performance. The present invention uses these measured density values to adjust density values in the input image file, without requiring knowledge of how an image forming assembly obtains a desired density. Therefore, the present invention minimizes the need for thorough technical understanding of the particular image forming assembly being used.
A further advantage of the present invention is that it minimizes the need for a printing apparatus manufacturer to obtain detailed information about internal operation of an image forming assembly. A printing apparatus manufacturer, when assembling a printing apparatus, can employ an image forming assembly from another supplier, without requiring detailed internal information on the image forming assembly.
A further advantage of the present invention is that it allows a printing apparatus manufacturer to obtain non-uniformity correction even if a printing apparatus comprises an image forming assembly that does not already have non-uniformity correction.
A further advantage of the present invention is that it improves upon any built-in uniformity correction already applied by an image forming apparatus manufacturer. There is, moreover, no need to interfere with or modify any uniformity correction that is supplied with the image forming apparatus. An improvement to performance can be effected without changing any existing, builtin non-uniformity correction.
A further advantage of the present invention is that it provides a method for compensation for non-uniformity that can be used independently from printing apparatus calibration. Uniformity adjustments are separately performed from calibration adjustments.
A further advantage of the present invention is that it allows a printing apparatus manufacturer to adapt a printing apparatus to use a different image forming assembly, allowing the design of a printing apparatus that is not constrained to using a specific image forming assembly.
Yet a further advantage of the present invention is that it obtains non-uniformity correction without further complicating the design of drive electronics for the printhead.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.